1. Field of the Invention
This invention relates to delay lines and, more particularly, to transmission line delay lines.
2. Description of Related Art
Transmission lines can be used as delay lines in many applications such as, for example, interference cancelling circuits used with radio transmitters and receivers, both fixed and mobile, and as delay lines in radar systems. In addition, such delay lines can also be used as signal storage media for sonar and electronic countermeasures equipment.
Radio transmission towers that simultaneously transmit and receive radio signals create interference in the received signals when the transmitter is transmitting. The method usual used to attenuate this interference is to sample the transmitted signal, invert the signal and add the inverted signal to the received signal to cancel out the interference.
While the interference arises from the coupling of the signal from the transmitting into the receiving antennas at the top of the tower, the circuitry to remove the interference is located at the base of the tower. Thus, the delay time for the signal to propagate from the transmitter up the tower to the transmitting antenna and back down again must be compensated for. Presently the signal delay is realized by passing the output signal from the transmitter through a length of transmission line that is approximately twice the height of the tower before the signal is inverted and added to the receiver input. The practical result is that a relatively large spool of costly cable sits at the base of the tower for each transmitter.
It can be appreciated that a circuit and method that can essentially half the length of cable required for transmission delay lines is highly desirable.